Same gear inches different gears

[Mikej]

I'm trying to figure out why it feels easier to use large large vs small small at the same gear inch. Gear inches calculate distance at the final output, but do not take into consideration if the height of the gears will offer mechanical advantages. Anybody know what I mean? Bigger gears have more leverage, I just need a formula.

[Grant McLean]

It's psychological, there is no difference.

-g

[christian]

Ok, not to get crazy, but bigger gears will have -slightly- less friction, because the rollers of the chain are making a less acute angle.

Of course, it'll weigh more, because you'll need more chain.

These are theoretical, not perceptible differences.

[Pete Mckeon]

Gear inches are gear inches no matter if if has a big rear or a small front. Differences are how we fell as we spin/peddle.

Thing that is in style now is a compact crank with a 34 tooth front and with Sram wifi 32 tooth in back---thus you have a comparable to shimano triple with 30 front tooth ring and a 28 tooth rear . I talk in regard to road riding.:)

I prefer triple for the rockies and for VT. but that is just me and finding triple selections is harder today

[Ralph]

It's psychological, there is no difference.

-g

Correct.....It's just basic math. (Physics....levers and pulleys) You could throw crank length into equation. But any mechaincal advantage you can get with longer cranks can be made up for with same percentage lower gear. That's why some say crank length has little (didn't say none) effect on power to rear wheel.

[Mikej]

Gear inches are gear in the fact that each revolution of the crank arm will yield the same distance travelled at the wheel. They do not take mechanical advantage into consideration. Think about the leverage of a larger gear its like this- imagine the Chain is a rope and you pull it - the larger gear will have a larger fulcrum offering mechanical advantage. Gear inches are only a distance calculation.

[christian]

Nonsense Mike. Time to do a bit of math. If the ratio is the same, the effort is the same. Size of chain wheel and sprocket have nothing to do with it.

[christian]

If that were true, they'd be riding tall bikes in the TdF, so they could fit 500t chain wheels.

[Mikej]

Anybody got a power meter that could check?

[Mikej]

If that were true, they'd be riding tall bikes in the TdF, so they could fit 500t chain wheels.

Good point

[Ralph]

If that were true, they'd be riding tall bikes in the TdF, so they could fit 500t chain wheels.

Well.....A 52X13 (4-1) is a 108" (approx....using 27 for wheel size) gear. A 500 Tooth front ring with a 125 cog in rear would be same gear. A 4-1 ratio. It's just math folks. The differences in mechaincal advantage would only come from the lever length and wheel size. But a 4-1 gear is a 4-1 gear, no matter how you juggle the cog and chainring sizes. If you want to take it a step or two further and talk about how far the bike travls in one crank revolution, then wheel size, circumference, comes into play. Again....just 7th grade math.

[Dave]

What takes "leverage" into account is the gear ratio. Gear inches is nothing but a gear ratio times a constant (27).

A 52/13 and a 48/12 will both have the same leverage and same gear-inch value.



http://sheldonbrown.com/gain.html

[Ralph]

What takes "leverage" into account is the gear ratio. Gear inches is nothing but a gear ratio times a constant (27).

A 52/13 and a 48/12 will both have the same leverage and same gear-inch value.



http://sheldonbrown.com/gain.html

Agree....Everything else being equal....52X13 and 48X12 is the same. (or 100X25, etc)

[roydyates]

Ok, not to get crazy, but bigger gears will have -slightly- less friction, because the rollers of the chain are making a less acute angle.

Of course, it'll weigh more, because you'll need more chain.

These are theoretical, not perceptible differences.
while these are familiar arguments, my perceptual/psychological sense has been the opposite. I feel my bikes pedal more efficiently in the equivalent-gear-inch small ring combos.

While this is merely my perception, I've wondered why it seems true. (Randonneuring provides a lot of time for inconclusive pondering of such possibilities.) I have two guesses:

1. On some bikes, you get a better chainline in small ring combinations? I ride rando bikes using a triple, and this seems believable for a triple when comparing the big and medium rings.

2. In big ring combinations, the RD jockey wheels are pulling hard on the return chain. Perhaps that tension causes additional friction losses in through the RD jockey wheels.

[Chance]

Gear inches are gear in the fact that each revolution of the crank arm will yield the same distance travelled at the wheel. They do not take mechanical advantage into consideration. Think about the leverage of a larger gear its like this- imagine the Chain is a rope and you pull it - the larger gear will have a larger fulcrum offering mechanical advantage. Gear inches are only a distance calculation.

Mike, you are confusing yourself a bit here because you are partly correct, but overall your conclusion is wrong. You are basically looking at this backwards. In the example you mention above, you are correct that pulling on the chain would create more leverage on a larger chainring, thus leading to more torque at the drive axle. However, when we pedal a bike, we create a given amount of torque at a given cadence so using a larger chainring will lead to the chain being pulled on (i.e. – tensioned) to a lesser degree. Chain tension with a larger chainring and cog therefore has to be less given the same amount of effort by the rider. This lesser amount of chain tension then translates to the same amount of rear-wheel torque because the rear cog is proportionally larger by “EXACTLY” the same amount as the chainring. As long as you compare identical inch-gears (assuming same size rear tire), making the chainring “X” percent larger requires the rear cog to also be “X” percent larger. And if so, chain tension will be 1/X lower. Basically if we double the chainring and cog size, chain tension will be half as much (give or take for minimal friction differences).

In summary, the only thing that is affected by “leverage” differences between big and small rings and cogs is chain tension and its associated load on bottom-bracket and rear-hub bearings. And those are so small it’s not worth much consideration when it comes to pedaling efficiency.

[Chance]

Ok, not to get crazy, but bigger gears will have -slightly- less friction, because the rollers of the chain are making a less acute angle.

Of course, it'll weigh more, because you'll need more chain.

These are theoretical, not perceptible differences.

Larger ring/cog sizes indeed lead to less roller angles at chainring and cog, but there are more rollers per pedal revolution to offset this advantage. And also to add to friction, larger ring/cog sizes move more chain per pedal revolution over the rear derailleur where the chain is folded back and forth; hence adding to more friction. There are offsetting advantages and disadvantages to both. Either way it's too small a difference to worry about.

[carpediemracing]

Usually due to chain line the big-big type combinations have a lot more friction. This is even more so if the chain was sized using Shimano's recommendations (big-big combo plus one pair of links) since the chain is under so much tension from the rear derailleur.

As an experiment put your bike in the big-big and pedal forward while holding the rear wheel off the ground. Then repeat after moving the chain to the small-small. There should be, on a well maintained drivetrain, a significant difference in the minimal effort required to move the pedals.

If you use the small-small type combination to get the same gear ratio it'll probably be easier to pedal. How much easier? Maybe a few watts. I saw 1 watt on my SRM when I was soft pedaling at low rpms, i.e. I could hear the freehub body clicking so it was all friction forces. I've seen as high as 15w on my bike while not applying power to the wheel (friction is due to bearing issues).

In the old days some racers would intentionally make the chain just a touch too long so it would hang in the small-small. The idea was that since you're not supposed to use that gear then you should make the chain as long as possible for the small-next-smallest-cog.

A few watts may seem marginal but for many riders, including me, 2 watts is over a 1% improvement in my normal avg wattage. If I'm on the limit then just a few watts is enough to drop me or not.

On the other hand it's true that smaller cogs have more friction. Smaller cogs get less round - think of what a (theoretical) one tooth cog would look like - it would have just one side, it simply couldn't be a circle. In the old days in the TTT the teams would use enormous chainrings so that they could use larger cogs. They'd spin a 56x15 for example. This way they could keep the chain line straight, use larger cogs, and, for the normally flat TTTs, the few extra links etc wasn't a bad price to pay.

[Ahneida Ride]

It always feels better in the bigger ring ...

[ultraman6970]

You are right in the leverage...

As bigger the cog the bigger the leverage, bigger leverage will be easier to move... is the cog the one that will give you the feel on how hard the gear will feel... the chairing doesn't have that big incidence but maybe in the way the cadence will feel.

In this case 48x12 should feel harder to move than 52x13...

A parenthesis ok? roadies generally dont feel too much the differences as trackers do due to the nature of their races, a roady needs more gear will just go and put another cog and thats it, in the track if you arent careful picking what is ok for you or the type of race you can get stuck with something way too heavy or something too light...

For example 47x14, 50/51x15 and 52/53x16 are almost the same gear but in a track sprint will feel totally different, specially in the last 50 meters.

52x16 will be easier to move in the track and will be easier to keep it moving at high cadence but in a sprint probably before the last 50 meters you wont be able to accelerate no more... in the road 53x16 its a super nice gear.

50x15 is the perfect balance between cadence and acceleration because you can continue accelerating with that.

47x14 feels like you are dragging the wheel big time but you can continue accelerating with that thing, but you have a trade off... you have to be strong to move it or the 14 will eat you alive specially when pace making at high cadence.

Enjoy.

What takes "leverage" into account is the gear ratio. Gear inches is nothing but a gear ratio times a constant (27).

A 52/13 and a 48/12 will both have the same leverage and same gear-inch value.



http://sheldonbrown.com/gain.html

[11.4]

This has been beaten to death in the track world, with the conclusion that there's no power advantage in one gearing alternative versus another. It's been torn apart mathematically, tested with powermeters, tested in labs, you name it. No difference.

However, one thing that is measurable is that the smaller chainring/smaller cog combination offers faster snap but is more fatiguing on the legs. Legs do better on anything but a short power interval (sprint, kilo, etc.) with larger chainring and larger cog, because it has a smoother transition which may not translate directly to power, but does translate into less fatigue for the legs. This is why madison riders typically ride a 52x16 or x15 while the same riders in a sprint will put on a 48x14 or x13. As speeds increase and gears get higher, anything that prevents jarring the muscles and decreases fatiguing is a benefit. I'd suggest that this is what you may be feeling as greater power. It isn't exactly large, but it's the only difference.

[Dave]

48/12 and 53/13 are both 4:1 gear ratios. Although the chain tension is 10% higher with the 48/12 there's no way it will make any significant difference in power output.

Sure it makes a difference if you make a small change to the gear ratio, but that's an apples and oranges comparision.

[Grant McLean]

For example 47x14, 50/51x15 and 52/53x16 are almost the same gear




Except they're not the same gear.
The explanation of why these combinations
feel different is that they are different size gears!

52x16 = 87.7
47x14 = 90.6

That's like 3% difference.

-g

[Ahneida Ride]

This has been beaten to death in the track world, with the conclusion that there's no power advantage in one gearing alternative versus another. It's been torn apart mathematically, tested with powermeters, tested in labs, you name it. No difference.

However, one thing that is measurable is that the smaller chainring/smaller cog combination offers faster snap but is more fatiguing on the legs. Legs do better on anything but a short power interval (sprint, kilo, etc.) with larger chainring and larger cog, because it has a smoother transition which may not translate directly to power, but does translate into less fatigue for the legs. This is why madison riders typically ride a 52x16 or x15 while the same riders in a sprint will put on a 48x14 or x13. As speeds increase and gears get higher, anything that prevents jarring the muscles and decreases fatiguing is a benefit. I'd suggest that this is what you may be feeling as greater power. It isn't exactly large, but it's the only difference.

I'll buy this one.

[cachagua]

Larger chainring and larger cog... has a smoother transition

Smoother transition from what to what? Can you say more?

[palincss]

It's psychological, there is no difference.

-g

Not quite true. Larger sprockets are more efficient than small ones. When you get down to something like Capreo's 9T sprocket the efficiency difference is really dramatic; not quite so breathtaking at 11T but the difference is still there.

Now whether that's what the OP is feeling, that's another story.

[Chance]

48/12 and 53/12 are both 4:1 gear ratios.


Dyslexic much? Fixed it for you.;)

"48/12 and 52/13 are both 4:1 gear ratios."

[Chance]

Not quite true. Larger sprockets are more efficient than small ones. When you get down to something like Capreo's 9T sprocket the efficiency difference is really dramatic; not quite so breathtaking at 11T but the difference is still there.

Now whether that's what the OP is feeling, that's another story.

Are you talking about mechanical efficiency or smoothness? Small chain cogs feel to rotate less smoothly due to “polygonal action”, first discussed on Serotta forum years ago. Link below. But this lack of rotation smoothness doesn’t necessarily mean lower mechanical power transmission efficiency as it affects watts out versus watts in. Seems like two different issues, although they may be related.

http://forums.thepaceline.net/showthread.php?t=21207

No doubt that a 9T small cog could add a lot of pulsation to the pedaling stroke in theory. Don’t know the magnitude but probably much more than an 11T small cog if the non-linear relationship RPS mentioned is correct. My understanding is that going from a 52T to a 50T causes very little difference compared to going from a 13T to an 11T, and that’s probably much less than going from 11T to 9T.

[palincss]

Pedal Power Probe Shows Bicycles Waste Little Energy
...
The researchers found two factors that seemed to affect the bicycle chain drive's efficiency. Surprisingly, lubrication was not one of them.

"The first factor was sprocket size," Spicer says. "The larger the sprocket, the higher the efficiency we recorded." The sprocket is the circular plate whose teeth catch the chain links and move them along. Between the front and rear sprockets, the chain links line up straight. But when the links reach the sprocket, they bend slightly as they curl around the gear. "When the sprocket is larger, the links bend at a smaller angle," Spicer explains. "There's less frictional work, and as a result, less energy is lost."

The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."

-- http://www.jhu.edu/news_info/news/home99/aug99/bike.html

Googling for "9 tooth sprocket efficiency" also retrieves a hit from Transactions of the American Society of Mechanical Engineers, Volume 18, p.1071 comparing 7, 8 and 9 tooth sprockets, finding several percentage points difference in efficiency between 7 and 9.

Tony Hadland's book _The Spaceframe Moultons_ includes on p. 40 a quote from Frank Berto, "The main problem with very small sprockets is 'chordal action'. The sprocket isn't completely round so the chain moves with a jerky action. The uneven movement amounts to 2 percent for a 16-tooth sprocket, 3 percent for a 13 tooth and 4 percent for an 11 tooth sprocket."

[Chance]

The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."

Seriously? How can this not be clear to them? It's common knowledge.

[Dave]

Seriously? How can this not be clear to them? It's common knowledge.

Easy to say with no supporting info. Most people would think that higher tension means more friction and a loss of efficiency.

[Chance]

Easy to say with no supporting info. Most people would think that higher tension means more friction and a loss of efficiency.

Well, maybe it's not as common knowledge after all. On the other hand it's one of those things that if it has to be explained, why bother.

Seriously though, it "should" be common knowledge to people of their technical background. That they don't know is amazing. It’s in text books and design manuals all over the place. Nothing anyone can’t find in a few minutes.

[Gummee]

The chain running over the gears sounds different between big and small rings. Could be part of what you're feeling is being supplemented by your ears. :confused:

M

[Dave]

Well, maybe it's not as common knowledge after all. On the other hand it's one of those things that if it has to be explained, why bother.

Seriously though, it "should" be common knowledge to people of their technical background. That they don't know is amazing. It’s in text books and design manuals all over the place. Nothing anyone can’t find in a few minutes.

You should explain it then. The engineers at John Hopkins couldn't:

"The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."

http://www.jhu.edu/news_info/news/home99/aug99/bike.html

FWIW, I'm a mechanical engineer too, but I don't believe everything I read, even when written by other engineers.

Jobst Brandt has stated that chain elongation (change in pitch) is the only factor to worry about when it comes to a chain wearing out cogs. I've proven that to be wrong, in my own chain wear tests.

It's possible to use a Campy chain for 6,000 miles and only have .15% elongation, yet wear steel cogs to the point that they will no longer mate with a new chain - you get new-chain skip. The same thing can be done in less than 4,000 miles (and with less elongation) using Ti cogs. If you use a single chain with a cassette for that long, the wear on the rollers will be quite large and the cogs wear-in to fit those smaller rollers. Put a new chain on that cassette and it will skip, just like it does when a fast-elongating chain (like Shiman or KMC) is left in use to about 1% elongation, instead of being tossed at .5% elongation. High chain tension played a part in my tests, since I use using a 53/39/28 crank, with a 12-25 cassette and spent a lot of time climbing the Colorado mountains, with the 28/21 and 28/19 being my most-used combinations, while climbing.

[merlincustom1]

"Sur la plaque!"

[Bostic]

For what it's worth, for me climbing in 30x19 (41.5") is easier than climbing in 39x25 (41") on my bike with a triple. It doesn't matter if I'm running an 11-25 and cross chaining the middle ring or running a Sram 11-28 so 39x25 in the 9th position. The granny ring seems to spin smoother, even on the tandem that has an 11-32 XT Cassette.

[Mark McM]

Pedal Power Probe Shows Bicycles Waste Little Energy
...
The researchers found two factors that seemed to affect the bicycle chain drive's efficiency. Surprisingly, lubrication was not one of them.

"The first factor was sprocket size," Spicer says. "The larger the sprocket, the higher the efficiency we recorded." The sprocket is the circular plate whose teeth catch the chain links and move them along. Between the front and rear sprockets, the chain links line up straight. But when the links reach the sprocket, they bend slightly as they curl around the gear. "When the sprocket is larger, the links bend at a smaller angle," Spicer explains. "There's less frictional work, and as a result, less energy is lost."

The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."

Unfortunately, this is another case of the right answer to the wrong question. Spicer tested with a single size front chainring and at the same cadence and power output, and he found that a larger rear sprocket was more efficient than a smaller rear sprocket. But since he used the same chainring, he actually tested two different gear ratios.

When we want to ride at a particular power and cadence, we don't get to choose the gear ratio - the terrain will determine the gear ratio for us. So we can't just decide to use a particular rear sprocket - we have to choose a front and rear gear combination.

Clair L. Walton and John C. Walton re-examined Spicer's raw data to see if the same gear ratio composed of either larger or smaller chainring/sprocket combinations were more efficient at a fixed power and speed. They had to interpolate the data, but their conclusion was:

"The surprising and counterintuitive result is that the smaller sprockets have greater estimated chain efficiency at constant vehicle velocity and applied power than the larger sprockets. Therefore, the increased efficiency from the higher chain tension is more important than the loss of efficiency from having the smaller sprocket. Clearly more experimental data using different chainring and sprocket combinations will be required to answer questions on chain efficiency definitively."

http://ihpva.org/HParchive/PDF/hp51-2001.pdf

[Chance]

You should explain it then. The engineers at John Hopkins couldn't:

"The second factor that affected efficiency was tension in the chain. The higher the chain tension, Spicer says, the higher the efficiency score. "This is actually not in the direction you'd expect, based simply on friction," he says. "It's not clear to us at this time why this occurs."

http://www.jhu.edu/news_info/news/home99/aug99/bike.html

FWIW, I'm a mechanical engineer too, but I don't believe everything I read, even when written by other engineers.



Dave, since you insist. It's simple math. Nothing more or all that complicated.

Yes, there is more friction with higher chain tension. That's a given. But so what? What they are not taking into account is that with higher chain tension given all other conditions the same there is more power being transmitted, right? So if you double chain tension, you are all of a sudden transmitting twice as much power (more or less) but while friction goes up, it's unlikely to double. Hence, since "EFFICIENCY" is calculated as a percent of power that is transmitted, it actually goes up even though friction also goes up.

If you think about it, as chain tension starts to be reduced towards "zero" then efficiency also has to go to zero because there is no power at all being transmitted. Friction of a freewheeling system may be very low but efficiency is zero. These guys should know better than to let higher "absolute" friction numbers confuse them about what is nothing more than a ratio (power out/power in).

And speaking of bikes specifically, as mentioned in the past by me and others alike, bikes make this even more so because the RD chain friction is independent of drive chain tension and therefore rider effort as long as he is using same gear and cadence. If he pedals harder and goes from 100 to 200 watts at same cadence the overall friction won't double. That's why efficiency goes up.

This has been discussed on this forum before and charts of test data posted so it seemed to me to be common knowledge.

[Chance]

For what it's worth, for me climbing in 30x19 (41.5") is easier than climbing in 39x25 (41") on my bike with a triple. It doesn't matter if I'm running an 11-25 and cross chaining the middle ring or running a Sram 11-28 so 39x25 in the 9th position. The granny ring seems to spin smoother, even on the tandem that has an 11-32 XT Cassette.

That's not that suprising to me since my experience is similar; albeit with slightly different gear ratios. When riding 30X19 (41.5") your chainline is much straighter than when riding 39X25 (41"). That in itself should make a greater difference in my opinion than whether the sprockets are about 25% smaller in size and chain tension proportionally higher. When riding my triple the granny gets used due to improved chainline a lot more often than other riders seem to use their granny ring. Personally don't save it as a bailout. Mine gets used a lot.

[Mikej]

Chain tension is only increased when torque is increased.

[Mark McM]

Chain tension is only increased when torque is increased.

That is incorrect. For a given pedaling torque, chain tension can go up as the chainring size goes down.

For example, say I have a triple chainring crank with a 52 tooth outer chainring and a 26 tooth inner chainring - the 52 tooth chainring has twice the pitch diameter of the 26 tooth chainring. The crank act as lever with the BB as the fucrum, and lever lengths equal to the crank length on one side of the fulcrum and the chainring pitch diameter on the other side of the lever. If I pedal with a given torque with the 26 chainring, then there will be twice the tension on the chain than there would be with the 52 chainring, due to the difference in the lever arm ratios.

The same power is transmitted, because although the chain has twice the tension as with the larger chainring, it is moving at half the speed. Since power = force x speed, twice the force at half the speed equals the same amount of power.

Which leaves us with the question of efficiency. Since efficiency goes up with chain tension, but down with decrease in sprocket size, is the same gear ratio more efficient with a small/small chainring/sprocket or a large/large chainring/sprocket combination? Unfortunately, there has been no definitive testing, but the data available so far seems to indicate that the small/small combination may be more efficient (at least as certain power outputs).

[Louis]

Just ride the d@mned bike.

If you feel a significant difference for the same gear ratio (assuming the same crank length and rear tire diameter) then chances are it's all in your head. If you think the difference truly is real, then ride what makes you feel better and/or faster.

[dave thompson]

http://imgs.xkcd.com/comics/duty_calls.png

[TPetsch]

I believe my "muscle memory" feels a difference between the equal gear ratios on the big and small chain rings on the crank, thus it may appear that the equal ratios feel different.

By this I mean when I'm in equal gear ratios be it on the large or small rings on the crank the next gear either up or down on the cassette feels very different, as it should because those will be different ratios. If I'm riding the big chain ring and I click up or down a gear it feels very different than when I'm on the small chain ring of an equal ratio and click up or down. The equal ratios are indeed the same but the approach and cadence your leg muscles take to get there are very different -going through different ratios to get to those equal ratios- and you may "Feel" like there is a difference once you get there.

This making any sense?

[Mark McM]

I believe my "muscle memory" feels a difference between the equal gear ratios on the big and small chain rings on the crank, thus it may appear that the equal ratios feel different.

By this I mean when I'm in equal gear ratios be it on the large or small rings on the crank the next gear either up or down on the cassette feels very different, as it should because those will be different ratios. If I'm riding the big chain ring and I click up or down a gear it feels very different than when I'm on the small chain ring of an equal ratio and click up or down. The equal ratios are indeed the same but the approach and cadence your leg muscles take to get there are very different -going through different ratios to get to those equal ratios- and you may "Feel" like there is a difference once you get there.

This making any sense?

None what so ever. The path that your feet/legs travel is the same, regardless - that is defined by circular motion of the cranks. The relative inertias/loads are the same if the final drive ratio is the same, so the applied loads are the same through out the pedal circle. The only possible difference is some small differences in drivetrain due to chordal action - but that is so small as to be negligible for most sprocket sizes. Your muscles will have no difference in load through out the pedal circle.

You're really over thinking this.

[Dave]

Dave, since you insist. It's simple math. Nothing more or all that complicated.

Yes, there is more friction with higher chain tension. That's a given. But so what? What they are not taking into account is that with higher chain tension given all other conditions the same there is more power being transmitted, right? So if you double chain tension, you are all of a sudden transmitting twice as much power (more or less) but while friction goes up, it's unlikely to double. Hence, since "EFFICIENCY" is calculated as a percent of power that is transmitted, it actually goes up even though friction also goes up.

If you think about it, as chain tension starts to be reduced towards "zero" then efficiency also has to go to zero because there is no power at all being transmitted. Friction of a freewheeling system may be very low but efficiency is zero. These guys should know better than to let higher "absolute" friction numbers confuse them about what is nothing more than a ratio (power out/power in).

And speaking of bikes specifically, as mentioned in the past by me and others alike, bikes make this even more so because the RD chain friction is independent of drive chain tension and therefore rider effort as long as he is using same gear and cadence. If he pedals harder and goes from 100 to 200 watts at same cadence the overall friction won't double. That's why efficiency goes up.

This has been discussed on this forum before and charts of test data posted so it seemed to me to be common knowledge.

You can transmit the same amount of power with a 50/25 or 30/15, but the chain tension is proportional to the size of the chainring and will be 1.67 times larger with the 30/15 combination, even though the gear ratios are the same 2:1. That's the kind of example that started this discussion - same gear ratio, but different sized chainrings and cogs. That that should be the type of comparison where more chain tension and friction is somehow more efficient with the SAME power being transmitted.

You're thinking only of the simplest case, where chain friction would be a large percentage of a very low power output and constitute a smaller percentage of a larger power output. The engineers doing the efficiency test wouldn't be perplexed about something that simple.

[Chance]

You're thinking only of the simplest case, where chain friction would be a large percentage of a very low power output and constitute a smaller percentage of a larger power output. The engineers doing the efficiency test wouldn't be perplexed about something that simple.

No, that's not the case at all. My techinical argument is not limited to the simplest case or most any case for that matter. The example in my post was the simplest example that came to mind that would communicate the principle involved in its simplest form so that nearly everyone could understand it. As you unload a chain drive towards zero power (hence zero chain tension) it generally has to become more inefficient due to fricition until the efficiency also reaches zero. But you don't have to go to that extreme by cherry picking my words. My post also stated that cutting the power in half (which reduces chain tension in half) doesn't reduce friction by more than half so it should be clear to "EVERYONE" that efficiency has to decrease.

To your other point, either way it really doesn't matter because both cases are arguing the same principle in a different context. The principle is exactly the same. Whether you hold the sprocket sizes constant and increase torque (which increases chain tension, power transmitted and efficiency at a given cadence) or you decrease sprocket sizes at a fixed power and cadence (which increases chain tension and efficiency) it's essentially the same exact argument using different constants. That's all that really changes. The broader principle remains. And any engineer with experience in chain drives should know that as common knowledge.

Just about the only time this doesn't apply is when chain tension becomes so high that other factors like deformation add to inefficiency. However, on a bike that's hardly ever the case. Not even with two strong riders on a tandem does chain tension become high enough for efficiency to decrease due to the added tension.

[Dave]

The case of transmitting the same amount of power with the same gear ratio, but different chain tensions is entirely different than transmitting different amounts of power to create different chain tensions. All you're saying is the delivering twice the power does not double the friction loss. That's easy to believe.

In my example, let's say that a 100 watts of power is delivered. If the efficiency is 95% with a 50/25 gear, then is would be something greater, using a 30/15 gear - according to the testers. That is NOT intuitive. Most people would say that the 30/15 gearing setup with the greater chain tension would decrease the efficiency, since more tension usually means more friction, not less. Most people would also think that the chain tension is the same, if the gear ratio is the same. I brought up that fact a few years ago and it raised a lot of doubts from the forum.

[Chance]

The case of transmitting the same amount of power with the same gear ratio, but different chain tensions is entirely different than transmitting different amounts of power to create different chain tensions. All you're saying is the delivering twice the power does not double the friction loss. That's easy to believe.

In my example, let's say that a 100 watts of power is delivered. If the efficiency is 95% with a 50/25 gear, then is would be something greater, using a 30/15 gear - according to the testers. That is NOT intuitive. Most people would say that the 30/15 gearing setup with the greater chain tension would decrease the efficiency, since more tension usually means more friction, not less. Most people would also think that the chain tension is the same, if the gear ratio is the same. I brought up that fact a few years ago and it raised a lot of doubts from the forum.

Dave, in my opinion those two examples are not entirely different. And the fact that their data (and just about everyone else’s who have tried measuring this effect when done correctly) confirm this trend makes this a pointless argument. The data speaks for itself and shows we shouldn’t always trust our intuition.

Personally prefer not to rely on “intuition” to the same degree that most people do. My opinions are usually based on theory and usually only after they have been confirmed in testing. My intuition or gut feeling usually takes a back seat.

All of these examples are based on the same equations. Depending on what riding conditions we choose to evaluate, some variables become constants and vice versa, but the overall trend should follow the same direction for the most part.

In your 50/25 versus 30/15 example above (assuming everything else is equal) the greater chain tension in the 30/15 gear combination will undoubtedly produce more friction, but only on a “PER LINK” basis (chain tension is roughly 166% of the 50/25 combination). But given the same cadence and power, the chain is only traveling at 60% the speed, right? So while “each” link has greater friction there are a lot fewer links being used per second of riding to contribute towards overall friction. This shouldn’t be intuition, it’s simple math. In this example unless friction per link increases by at least 166% while riding in the 30/15 gear, the overall efficiency can’t help but go up (excluding bearing losses and the like).

[potatochip]

Not to bump a dead thread, but bump. I'm still spinning over what I've read. I can't seem to figure out whether a 50/36 would be more efficient than a 53/39. Walton suggests yes but Berto suggests no in expanding on Spicer. Any smarter people have a guess?


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[palincss]

Ok, not to get crazy, but bigger gears will have -slightly- less friction, because the rollers of the chain are making a less acute angle.

Of course, it'll weigh more, because you'll need more chain.

These are theoretical, not perceptible differences.

Actually, the weight of the drive train remains constant no matter which gear combination you shift into. :)

[christian]

I was thinking of two fixed-gear drivetrains, not two gears on the same drivetrain...

[Gummee]

Anecdotal evidence says I like the way the big ring and bigger cogs feel better than the small ring and smaller cogs.

As with anything: YMMV

M

edited to add: 53x16 had to be one of my favorite gears at the track until it got to the end of the night. Then the 50x15 was 'it.

[ultraman6970]

53x16 and 50x15 are almost the same gearing, the problem in the track in specific is that with 53x16 you can spin that quite quick but you can't drag it more, i mean... gain more speed out of it, you get to top speed and you cant squeeze more out of it. For a persuit is ok because you can pace it fine and quick, but in a sprint you lack of a final rush at the last 10 to 25 meters.

At the oposite you dont have that problem with the 50x15 at all, you can continue accelerating that thing because has more drag, my fav in per points races was 50 or 51x15... in something like elimination races you have to spin constantly fast and the 53 was more than enough, the problem was to dispute the final sprint with the 53x15.

47x14 is pretty much the same, not much torque in the 14, you have to be strong to move that, too much drag IMO but many trackers like it, specially sprinters.